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Last Updated: Tuesday, 29 June, 2004, 18:07 GMT 19:07 UK
Rice yields dip as planet warms
Worker in a paddy field, BBC
There is a strong link between increasing night temperatures and decreasing rice yields
Global warming could have a severe effect on rice production, say scientists working in the Philippines.

The researchers studied 12 years of rice yields and 25 years of temperature data, to work out how they are linked.

Yields dropped by 10% for each degree of warming, an alarming trend since rice is the staple diet for most of the world's expanding population, they say.

The study, by an international team, appears in the Proceedings of the National Academy of Sciences journal.

Increasing demand

World rice production must increase by about 1% every year, to meet the demand of our planet's bulging population.

Many experts believe the only way to achieve this output is to improve the productivity of existing cropland. The alternative - planting more crops - would involve a costly encroachment into natural ecosystems.

But can rice crops rise to this challenge? If the new research is correct, maybe not.

A team of researchers went to the International Rice Research Institute in the Philippines, to monitor the performance of rice crops planted next to a weather station.

Because the rice and the weather station were such close neighbours, it was possible for the researchers to relate varying performance with varying temperature.

They found that average daytime temperatures, which increased by 0.35C since 1979, had little effect on productivity.

However, there was a strong link between increasing night temperatures - which rose by an impressive 1.1C over 25 years - and decreasing rice yields, they discovered.

Hot nights

The authors believe this is because, during hot nights, rice puts more energy into respiring and less into growing.

"Increases in temperature, with all else being equal, can add to maintenance and respiration costs," explained co-author Kenneth Cassman, of the University of Nebraska, US.

Increases in temperature can add to the maintenance and respiration costs of rice
Kenneth Cassman, University of Nebraska, US
But why do night-time temperatures have such a big impact?

"There could be two things at work here," explained Professor Cassman. "It could be that the increase in temperature was too small during the day for one to see a difference in yield.

"Or it could be that temperature tends to increase on sunny days, and that leads to higher rates of photosynthesis [which accelerate growth]. So there is a confounding factor."

Jaded view

The team fear that global warming will make it increasingly difficult to feed the Earth's growing population.

Computer models of climate change suggest that night-time temperatures will continue to rise faster than in the day - by several degrees C in the coming decades.

This is bad news for rice because it often grows in the tropics - very near the top end of its temperature range. So a slight increase in temperature can bear a heavy cost.

"Our data suggests that where rice is grown near the upper end of its temperature range, a small increase in temperature can make it exceed that range - and yields suffer," said Professor Cassman.

Of course, as global warming renders some areas inhospitable to rice growth it will make other - cooler - regions more hospitable. But that should not necessarily be a comfort, Professor Cassman argues.

"You hear people arguing that warming effects really are not as severe as they appear to be, because you can simply move the locus of production south to north," he said. "But that is a rather jaded view, particularly if you don't live in the south, if you are not a poor farmer that depends on rice for their livelihood.

"So while we can call that an option we have to be very careful that it doesn't come at a price."


SEE ALSO:
Rice producers in 'cartel' talks
09 Oct 02  |  Business
GM rice: A growing Philippines debate
26 Sep 03  |  Asia-Pacific
Global warming 'biggest threat'
09 Jan 04  |  Science/Nature


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